1. Field of the Invention
This invention pertains generally to imaging, and more particularly to ultrasound imaging using a synthetic aperture ultrasound ray tomography and ultrasound waveform tomography.
2. Description of Related Art
Breast cancer is the second-leading cause of cancer death among American women. The breast cancer mortality rate in the U.S. has been flat for many decades, and has decreased only about 20% since the 1990s. Early detection is the key to reducing breast cancer mortality. There is an urgent need to improve the efficacy of breast cancer screening. Ultrasound tomography is a promising, quantitative imaging modality for early detection and diagnosis of breast tumors.
Ultrasound waveform tomography is gaining popularity, but is computationally expensive, even for today's fastest computers. The computational cost increases linearly with the number of transmitting sources.
Synthetic-aperture ultrasound has great potential to significantly improve medical ultrasound imaging. In a synthetic aperture ultrasound system, ultrasound from each element of a transducer array propagates to the entire imaging domain, and all elements in the transducer array receive scattered signals.
Many conventional ultrasound systems record only 180° backscattered signals. Others are configured to receive only transmission data from the scanning arrays. Accordingly, these systems suffer from extensive computational costs, insufficient resolution, or both.
It is difficult to reconstruct the region far away from an ultrasound transducer array when using reflection data for ultrasound waveform tomography. The geometrical spreading is the primary cause of this problem. The defocusing effect in synthetic-aperture ultrasound is stronger in the region far away from a transducer array than the region close to the transducer array. This defocusing effect may play a role in ultrasound waveform tomography using reflection data.
Preconditioning the gradients has been introduced to accelerate the convergence of waveform inversion. It has been shown that the diagonal terms of the approximate Hessian is a zero-lag correlation of the scattered waves, which represent the geometrical spreading effects as the scattering points move away from the sources and receivers. It has been suggested to scale the gradient by the diagonal terms of the approximate Hessian. However, it is generally expensive to calculate Jacobin matrix. One method is to replace the zero-lag autocorrelation of Green's functions by a pseudo-Hessian matrix. This approach reduces the computational cost by assuming that the zero-lag autocorrelations of the Green's functions are the same. Others have introduced a different pseudo-Hessian matrix using the amplitude of impulse responses from the sources to approximate the zero-lag autocorrelations.